In this study, we address cellular mechanisms that: 1) promote cytoplasmic sequestration of wild type p53 family proteins, thus deactivating them and 2) defeat cytoplasmic sequestration of wild type p53 family proteins, thus reactivating their apoptotic function in cancer cells. We take advantage of a naturally occurring leukemia in the soft-shell clam, Mya arenaria, in which clam homologs that we have identified for human p53 and p63/73 proteins are rendered non-functional by their sequestration in the cytoplasm of leukemic clam hemocytes. This phenomenon in clams reflects that seen in a subset of human cancers and is inducible in mouse NIH 3T3 cells. In all of these cases, wild type p53 does not function because it is retained in the cytoplasm. Inactivation of p53 through cytoplasmic sequestration has not always been evaluated and may be a more widespread cause for malfunction of p53 in human cancers than has previously been anticipated. In deuterostomes, there are no studies of inactivation of p63 or p73 proteins by cytoplasmic sequestration. Experiments in our laboratory have demonstrated that a correlation exists between the defeat of cytoplasmic sequestration of p53 and p63/73 proteins and the cytotoxicity and apoptosis of leukemic clam hemocytes after treatment with the topoisomerase II inhibitors - etoposide, doxorubicin and mitoxantrone. Our data suggest that p53- or p63/73-dependent mechanisms involved in cancers in these distantly related species may be very similar despite the 500MY that have intervened since the deuterostome and protostome lines of evolution diverged from each other. Clam leukemia has structural and molecular similarity to human cancers (e.g., Burkitt's and Burkitt-like lymphomas) and provides an easily accessible, naturally occurring, comparative, in vitro and in vivo model for examining mechanisms that promote or defeat sequestration of p53 and p63/73 proteins in the cytoplasm and thus extend hemocyte life span. In the present study, we will use this novel non-mammalian model to address two specific aims: I) To determine if mechanisms that result in sequestration of p53 and p63/73 proteins in the cytoplasm of clam (a protostome) leukemic hemocytes are similar to or different from those recognized for humans and other deuterostomes and II) To determine if treatment with topoisomerase II inhibitors leads to overexpression of clam p53 and p63/73 proteins. Overexpression might overwhelm mechanisms that sequester these proteins in the cytoplasm of leukemic clam [unreadable] hemocytes and lead to a resumption of clam p53 and/or p63/73-induced apoptosis. [unreadable] [unreadable]